Creepless snap-acting thermostatic switch



May 6, 1969 G. E. WEHL. ET AL CREEFLESS SNAPACTING THERMOSTA'IIC SWITCHFiled May 16, 1967 Fig. 6 I I4 2 8 INVENTORS Glenn E. Wehl Thomas EWelker BY WUZJM ATTORNEY United States Patent 3,443,259 CREEPLESSSNAP-rating}? THERMOSTATIC "r SWI Glenn E. Wehl and Thomas E. Welker,North Canton, Ohio, assignors to Portage Electric Products, Inc., NorthCanton, Ohio, a corporation of Ohio Filed May 16, 1967, Ser. No. 638,818Int. Cl. H01h 61/0], 37/54, 37/12 US, Cl. 33789 7 Claims ABSTRACT OF THEDISCLOSURE BACKGROUND This invention relates to small, inexpensivemass-produced thermostatic switches such as are used for overloadprotective purposes with small electric motors, home appliances, etc.US. Patent No. 2,820,870 to Moksu shows a thermostat of this generaltype, in which an effort is made to secure positive quick-opening (andclosing) action by providing a dish-shaped portion in a bimetallictemperature-sensitive arm. In the fabrication and mass assembly of smallsnap blades it is extremely difficult to prevent such an assembly frombeing subject to creep, i.e., a tendency for the electric contacts toseparate slightly before the snap action occurs. Such creep action isdestructive to the contacts and destroys the intended beneficial effectof the snap action, which is to rapidly separate the electric contactswhen a predetermined temperature has been reached, so as to minimizearcing and extend contact life, as well as to provide accurate,consistently repeatable operation. The present invention overcomes orgreatly reduces the creep tendency as well as providing a very simpleand reliable thermo static switch which lends itself to rapid andinexpensive assembly and production.

SUMMARY In accordance with the present invention, a bimetalliccontact-carrying switch arm is provided with a dished portion for snapaction. This arm is supported at one end by a first terminal strip, ofheavier and more rigid construction than the bimetallic arm, and carriesan electric contact at the other end, which contact, in the closedposition of the switch, engages and makes electrical contact with arigid terminal strip. At some predetermined temperature, the snap dishflexes into its snap position (in which its concave side becomesconvex), causing the free end of the bimetallic strip to flex away fromsaid rigid terminal with which its cont-act had been engaged. A rigidfulcrum, preferably formed as a part of the first terminal strip andextending from it toward and in engagement with the bimetallic strip ata point toward the free end of the bimetallic strip and beyond at leastthe major portion of the dish-shaped snap portion, is at all times inphysical contact with the bimetallic strip so that the bimetallic stripflexes about its point of engagement with said fulcrum. By virtue ofthis construction the electrical contacts remain fully and positivelyengaged as the temperature increases until the dish snaps, at which timethe electrical contacts are very rapidly separated. The reverse actionalso occurs, i.e., on cooling, the contacts close suddenly, althoughthis is normally less of a problem in such switches.

The specific nature of the invention, as well as other objects andadvantages thereof, will clearly appear from a description of apreferred embodiment as shown in the accompanying drawing, in which:

FIG. 1 is a sectional view of a switch according to the invention, takenon line 1-1 of FIG. 2;

FIG. 2 is a similar sectional view taken on line 22 of FIG. 1;

FIG. 3 is an end view of the device shown in FIG. 1;

FIGS. 4, 5 and 6 are perspective views showing individual components ofpart of the switch assembly;

FIG. 7 shows the components of FIGS. 4, 5 and 6 assembled together withthe bimetallic unit; and

FIG. 8 is a view similar to FIG. 1, but showing an alternative form ofconstruction.

As shown in FIGS. 1 and 2, the thermostatic switch is housed in andsupported by a casing 2, preferably in the for-m of drawn metal tube ofgenerally rectangular shape, although in some instances the tube may beomitted and the thermostat suitably located for the type of protectiondesired. The entire thermostat assembly 3 comprises, in addition to thehousing 2, an insulating retaining subassembly of rigid elements 4 and7, which interlock to retain between them a rigid terminal arm 6, asbest shown in FIGS. 1 and 3. On top of this subassembly, and between theupstanding arms of insulating member 4, is placed terminal arm 3, whichat its exposed end is shaped similarly to terminal arm 6, so that whenthis entire switch assembly is slid into the tubular casing 2, it isrigidly held against displacement or relative movement of the parts, dueto the interlocking construction. However, as the purpose is merely toretain elements 6 and 8 in the relative position shown in the figures,it will be apparent that other suitable insulating supporting meanscould be employed. After the assembly has been completed as shown inFIG. 1, the elements may be firmly locked into place by distorting thecasing so as to exert pressure between the top and bottom sides, thisbeing a well-known technique as shown, for example, in US. Patent No.3,223,808 to Wehl, for retaining a switch assembly in a metal casing.

As previously stated, lower terminal arm 6 is made rigid and immovablein the assembly. The upper terminal arm 8 may also be similarly rigid,although in some instances it may be permitted a small amount offlexibility if desired. Arm '8 carries at its end within the casing, thebimetallic strip 9, to the end of which it is secured in any suitablefashion so that arm 8 and strip 9 form a unitary subassembly. Bimetallicstrip 9 has a dished-out portion 11 preferably located fairly close tothe center thereof intermediate its ends, and carries at its free end asuitable electrical contact 13 which is normally in electricalengagement with terminal arm 6. If desired, a suitable electricalcontact may also be provided on arm 6 for engagement with contact 13. Afulcrum 12 extends out from arm 8 into contact with bimetallic strip 9at a point close to the dished-out portion 11, and between thisdished-out portion and contact 13. It is possible for the fulcrum pointto engage strip 9 somewhat within the confines of the dished-outportion, but if so this should be quite close to the edge of thedished-out portion, so that when, as the bimetallic strip 9 heats up,the dishedout portion 11 assumes the shape shown in FIG. 7, in which itspreviously concave side becomes convex, the fulcrum point will at alltimes remain in contact with the bimetallic member; obviously, this isnot possible if the fulcrum point 12 extends too far into the dished-outportion, since in that case the sudden flexure of this portion wouldcause contact with the fulcrum point to be lost. To avoid this, thefulcrum should be in contact with the bimetallic element well beyond thecenter point of the dish-shaped portion and toward the contact end.

FIG. 7 shows the thermostatic subassembly in the switch-open position,after the dished-out portion 11 has flexed. In this position, anelectrical circuit controlled by the thermostatic switch will obviouslybe opened, and since the switch flex is due to a condition ofoverheating, the electrical equipment will be given a chance to cooldown, which is the'manner in'which such thermostatic switches areusually employed.

Due to the position of the fulcrum point 12, it will be apparent thatflexure of the dishedout portion .11 causes a very large and rapidmovement of the contacts, and therefore they can be initially so firmlybiased into engagement that there is very little possibility of anycreep occurring right up to the moment when the dish snaps.

In order to set the initial calibration of the switch so that it willopen at a desired'point, the case may be distorted by external pressureas shown at 14, as is also well-known in the art, andshown, for example,in the above-mentioned patent 'to Wehl, No. 3,223,808.

FIG. 8 shows an alternative form of construction, in which parts whichare the same as those shown in the previous modification bear the samereference numerals as in FIG. 1, while corresponding parts which arealtered on construction bear the same reference numeral'with a primeadded. It'will be seen that the only difference between the twoembodiments is that the bimetallic strip 9 is supportedon terminal arm 8in .a reverse position relative to FIG. 1, and the fulcrum point 12! isprovided at the end of arm 8', instead of being intermediate its ends.The lower rigid terminal arm 6' is necessarily made longer than inFIG.L1, in order to engage the Contact at the end of the bimetallicstrip, but'otherwise the operation is the same. The flex position of thebimetallic arm 9 is indicated by dotted-'lines. It will be apparent thatthe operation of the modification shown in FIG. 8 is essentially thesame as that shown in FIG. 1.

It'will be apparent that the embodiments shown are only exemplary'andthat various modifications can be made in construction'andarrangement within the scope of our invention as defined in the appendedclaims.

1. Thermostatic snap switch construction comprising:

(a) an insulated mounting member,

(b) first and second terminal arms supported by said mounting member andextending outtherefrom in spaced, substantially para'llelrelation, thesecond of said arms being a rigid member,

(c) a bimetallic arm parallel to and spaced from said first terminalarm, the" other end being a free end,

(d) a dished-shaped-thermal snap portion intermediate the ends of saidbimetallic arm, said snap portion having two positions at differenttemperatures,

(e) a fulcrum point element extending toward and in contact with saidbimetallic arm at a point toward the other endof'said bimetallic armfrom the fixed end thereof and beyondthe center point of said dishshapedportion,

(f said bimetallic arm extending still further beyond said fulcrum pointand bearing an electrical contact near its first end, saidelectricalcontact being electrically engaged with said rigid second arm in onethermal position of said thermal snap portion and being disengaged fromcontact with said rigid arm in the other thermal position of said snapportion as the bimetallic arm flexes about said fulcrumpoint,

(g) said fulcrum point remaining in contact with the bimetallic arm inboth thermal positions.

2. The invention according to claim 1, said fulcrum point extending fromand being supported by said first arm. I

3. The invention according to claim 2, at least a por tion of said firstarm having some flexibility toward and away from the second arm, butbeing less flexible than said bimetallic arm.

4. The invention according to claim 2, and a closed metallic casingsupporting and enclosing said thermo static snap switch.

5. The invention according to claim 1, said bimetallic arm being fixedat said one end to the end of said first arm remote from said mountingmember.

6. The invention according to claim 1, said bimetallic arm being fixedto said first arm at a point intermediate its ends.

7. The invention according to claim 4, said mounting member comprisingan assembly of two interlocking members of rigid insulating material,said terminal arms having cut-out portions near the mounted endsthereof, so arranged that the entire assembly is interlocked immovablywhen assembled in said casing.

References Cited UNITED STATES PATENTS 2,820,870 1/1958 Moksu 337-3683,104,296 9/1963 Moksu et al. 337-89 3,194,924 7/1965 Moksu et al.337-89 BERNARD A. GILHEANY, Primary Examiner. R. L. COHRS, AssistantExaminer.

US. 01. X.R.

